This invention relates to hydrofoils, specifically to such devices which are used to create directional movement relative to a fluid medium, and this invention also relates to swimming aids, specifically to such devices which attach to the feet of a swimmer and create propulsion from a kicking motion.
One of the major disadvantages which plague prior fin designs is excessive drag. This causes painful muscle fatigue and cramps within the swimmer""s feet, ankles, and legs. In the popular sports of snorkeling and SCUBA diving, this problem severely reduces stamina, potential swimming distances, and the ability to swim against strong currents. Leg cramps often occur suddenly and can become so painful that the swimmer is unable to kick, thereby rendering the swimmer immobile in the water. Even when leg cramps are not occurring, the energy used to combat high levels of drag accelerates air consumption and reduces overall dive time for SCUBA divers. In addition, higher levels of exertion have been shown to increase the risk of attaining decompression sickness for SCUBA divers. Excessive drag also increases the difficulty of kicking the swim fins in a fast manner to quickly accelerate away from a dangerous situation. Attempts to do so, place excessive levels of strain upon the ankles and legs, while only a small increase in speed is accomplished. This level of exertion is difficult to maintain for more than a short distance. For these reasons scuba divers use slow and long kicking stokes while using conventional scuba fins. This slow kicking motion combines with low levels of propulsion to create significantly slow forward progress.
Much of the drag created is due to the formation of turbulence around the blade portion of the fin. This turbulence occurs because prior fin designs do not adequately address the problem of flow separation and induced drag while lift attempting to generate lift. This destroys efficiency and severely reduces lift. On an airplane wing for instance, Bernoulli""s principle explains that the air flowing over the convexly curved upper surface must travel over a greater distance than the air flowing underneath the lower surface of the wing. As a result, the air flowing over the upper surface must travel faster than the air flowing underneath the wing in order to make up for the increase in distance. Because of this, the air pressure along the upper surface of the wing decreases while the air pressure underneath the lower surface of the wing remains comparatively higher. This difference in pressure between the upper and lower surfaces of the wing causes xe2x80x9cliftxe2x80x9d to occur in the direction from the lower surface towards the upper surface. Because of this pressure difference, the lower surface on an airfoil is called the high pressure surface, while the upper surface is called the low pressure surface.
Another way of creating lift is to very the angle of attack. This is the relative angle that exists between the actual alignment of the oncoming flow and the lengthwise alignment of the foil (or chord line). When this angle is small, the foil is at a low angle of attack. When this angle is high, the foil is at a high angle of attack. As the angle of attack increases, the flow collides with the foil""s high pressure surface (also called the attacking surface) at a greater angle. This increases fluid pressure against this surface. While this occurs, the fluid curves around the opposite surface, and therefore must flow over an increased distance. As a result, the fluid flows at an increased rate over this opposite surface in order to keep pace with the fluid flowing across the attacking surface. This lowers the fluid pressure over this opposite surface while the fluid pressure along the attacking surface is comparatively higher. Because of this pressure difference, the attacking surface is the high pressure surface and the opposite surface is called the low pressure surface or lee surface.
The increase in pressure along the high pressure surface combines with the decrease in pressure along the low pressure surface to create a lifting force upon the foil. This lifting force is substantially directed from the high pressure surface towards the low pressure surface. Varying the foil""s angle of attack in this manner is important in swim fin designs because it enables lift to be generated on both the upstroke and the down stroke of the kicking cycle.
Although this method of generating lift is commonly used on prior swim fin designs, many problems occur that significantly reduce performance. One problem is that prior designs place the propulsion foil at excessively high angles of attack. In this situation, the flow begins to separate, or detach itself from the low pressure surface of the foil. When this occurs, the foil begins to stall. The separated flow forms an eddy which rotates around a substantially transverse axis above the low pressure surface. This eddy causes the fluid just above the low pressure surface to flow in a backward direction from the trailing edge towards the leading edge. This separation decreases lift since it reduces the amount of smooth flow occurring over the low pressure surface. This is a serious problem because smooth flow must exist in order for lift to be generated efficiently.
When the angle of attack becomes too high, the foil stalls completely and the flow along the low pressure surface separates into chaotic turbulence. This destroys lift by preventing a strong low pressure zone from forming over the low pressure surface, or lee surface. As a result, only a small difference in pressure exists between the opposing surfaces of the foil. Many prior fin designs suffer from this problem because they employ a horizontally aligned blade which is kicked vertically through the water. In this situation, the angle of attack is substantially close to 90 degrees, and therefore the blade is completely stalled out. This causes the blade to act more like an oar blade or paddle blade rather than a wing.
As well as destroying lift, stall conditions also cause high levels of drag. When areas of laminar flow (a flow condition where fluid passes over an object in a series of undisturbed layers) are abruptly converted into chaotic turbulent flow, a high drag condition known as transitional flow occurs. Because prior swim fin designs create stall conditions and chaotic turbulence along their low pressure surfaces, they generate high levels of drag from transitional flow.
Another problem that occurs at higher angles of attack is the formation of vortices along the outer side edges of the blade which cause induced drag. The difference in pressure existing between the attacking surface and the low pressure surface causes the fluid existing along the blade""s attacking surface to flow outward toward the side edges of the blade, and then curl around the outer side edges toward the low pressure surface. As this happens, the swirling motion creates a streamwise tornado-like vortex along each side edge of the blade just above the blade""s low pressure surface. As the water curls around the side edges of the blade, these vortices carry the water in an inward direction along the low pressure surface. After this happens, the vortices curl the water in a downward direction against the blade""s low pressure surface. As this water is forced downward against the low pressure surface, it is moving in the opposite direction of desired lift thereby further reducing lift. This downward moving flow deflects the fluid leaving the trailing edge at an undesirable angle that is oppositely directed to the direction of desired lift. Because the direction of lift is perpendicular to the direction of flow, this downward deflected flow (called downwash) causes the direction of lift to tilt in a backward direction. Consequently, a significant component of this lifting force is pulling backward upon the blade in the opposite direction of blade""s movement through the water. This backward force is called induced drag. Induced drag becomes greater as the blade""s angle of attack is increased. Because prior designs typically use extremely high angles of attack, they experience high levels of induced drag.
In addition to increased drag, the downward deflected flow (downwash) behind the trailing edge significantly decreases the blade ""s effective angle of attack which further reduces lift. As the flow behind the trailing edge is deflected downward (in the opposite direction of the lifting force) the angle of attack existing between the blade and this downward deflected flow (called the induced angle of attack) is less than the angle of attack existing between the blade and the oncoming flow (called the actual angle of attack). This reduces the blade""s ability to create a significant difference in pressure between its opposing surfaces for a given angle of attack. This creates a significant decrease in lift on the blade.
The induced drag vortex also decreases performance by further decreasing the pressure difference between the opposing surfaces of the blade. As the water escapes sideways around the side edges of the blade, it expands in a spanwise direction along the blade""s attacking surface. This decreases pressure along this surface, thereby decreasing lift. Also, because a substantial portion of the water flowing along the attacking surface is traveling in a more sideways direction and less of a lengthwise direction, this water is less able to assist in creating forward propulsion.
In addition, the high speed rotation of the vortex creates centrifugal force which evacuates fluid away from the center of each vortex (the vortex core). This creates a large decrease in pressure within the vortex core. The decreased pressure within this core is lower than the low pressure zone originally created along the low pressure surface by the foil""s angle of attack. As a result, this new low pressure zone increases the rate at which water flows around the side edges away from the high pressure surface and toward the low pressure surface. This further decreases the pressure within the high pressure zone existing along the attacking surface. Because this reduces the overall pressure difference occurring across the blade, lift is significantly reduced.
As the vortex forces this outwardly escaping fluid down upon the blade""s low pressure surface, fluid pressure is increased along this surface. This decreases lift by decreasing the difference in pressure occurring between the opposing surfaces of the blade. The swirling motion of each vortex also prevents water from flowing smoothly over a significant portion of the blade""s low pressure surface. This decreases lift by preventing the blade from forming a strong low pressure center along a substantial portion of its low pressure surface. In addition, this disturbance within the flow over the low pressure surface (created by the induced drag vortex) can cause the blade to stall prematurely.
The problems associated with induced drag vortex formation increase as the blade""s aspect ratio decreases. Aspect ratio can be described as the ratio of the blade""s overall spanwise dimensions to its lengthwise dimensions. A blade that has an overall spanwise dimension that is relatively small in comparison to its overall lengthwise dimension, is considered to have a low aspect ratio. Low aspect ratio foils tend to produce stronger induced drag vortices, and are therefore highly inefficient.
Low aspect ratio blades are commonly found in prior swim fins which are used separately by each foot in a scissor-like kicking motion. The spanwise dimensions are limited in these designs in order to prevent the blade on one foot from colliding with the blade on the other foot during use. In this situation, the only way to increase the blade""s surface area is to further increase the blade""s lengthwise dimensions. This further reduces the blade""s aspect ratio and increases induced drag.
Prior fin designs do not provide effective methods for reducing induced drag type vortices. Many designs use vertical ridge-like members which run substantially parallel to the lengthwise fin""s center axis, and extend perpendicularly from at least one surface of the blade. The purpose is to encourage aftward flow, reduce spanwise flow, and stiffen the blade. However, these devices do not adequately reduce spanwise flow or induced drag type vortices. Moreover, these devices create additional drag of there own.
Another problem with prior fin designs is that they exhibit severe performance problems when they are used for swimming across the surface of the water. While kicking the fins at the water""s surface, they break through the surface on the up stroke, and then on the down stroke they xe2x80x9ccatchxe2x80x9d on the surface as they re-enter the water. Before the fin re-enters the water, it moves freely through the air and gains considerable speed. As the fin re-enters the water, a majority of the blade""s attacking surface is oriented parallel to the water""s surface. As a result, the blade slaps the surface of the water and its downward movement is abruptly stopped. This instantaneous deceleration creates high levels of strain for the user""s ankles and lower leg muscles. Because downward movement ceases upon impact with the water, the strong downward momentum generated while the swim fin moves through the air (above the surface) is wasted and is not converted into forward propulsion after re-entering the water.
After this impact with the water""s surface has occurred, the fin is slow to regain movement under water because of severe drag. This lag in time that occurs on the down stroke prevents the user from attaining fully productive kicking strokes. Before the downward moving fin is able to regain enough speed to begin effectively assisting with propulsion, it must be lifted out of the water again because the other fin (which is on its upstroke) has already broken the water""s surface and is ready to begin its down stroke. Because it is difficult to kick both feet in an unsynchronized manner, this situation is awkward, strenuous, irritating, and highly inefficient. Over large distances, this problem can create substantial fatigue. This is particularly a problem for skin divers, body surfers, and body board surfers who spend most of their time kicking their fins along the water""s surface. It is also a problem for SCUBA divers who swim along the surface to and from a dive site in an attempt to conserve their supply of compressed air. Fatigue and muscle strain to SCUBA divers during surface swims is particularly high because prior SCUBA type fins have significantly long lengthwise dimensions. This causes increased levels of torque to be applied to the diver""s ankles and lower legs as the blade slaps the surface of the water. Because such longer fins create high levels of drag from a decreased aspect ratio, prior SCUBA type fins are significantly slow to re-gaining downward movement after catching on the water""s surface. Even below the surface, such prior fins offer poor propulsion and high levels of drag which severely detract from overall diving pleasure.
Both U.S. Pat. Nos. 169,396 to Ahlstrom (1875), and 783,012 to Biedermann and Howald (1906) use two parallel propulsion blades which are mounted beneath the soul of the foot. The design is intended to be used with forward and backward kicking strokes along a horizontal plane. This stroke is awkward and extremely inefficient. Each of the parallel blades pivot along a lengthwise axis that extends parallel to the sole of the swimmers foot. The blades swing closed to a zero degree angle of attack on the forward stroke, and then swing open to about a 90 degree angle of attack on the backward, or propulsion stroke. This fin design attempts to gain propulsion from a pushing motion rather that a kicking motion. Both designs produce high levels of drag on the propulsion stroke and are not appropriate for use with contemporary vertical kicking strokes.
U.S. Pat. No. 2,950,487 to Woods (1954) uses a horizontal blade mounted on the upper surface of the foot which rotates around a transverse axis to achieve a reduced angle of attack on both the upstroke and the down stroke. The blade has a deep V-shaped cut down the center of the blade which divides the blade into a left half and a right half. These two sections are connected by a narrow strip of blade section running between them at the apex of the V-shaped cut out. Both left and right blade halves are fixed to each other within the same plane and no system is used to encourage any portion of these halves to flex, twist, or rotate in a way that can significantly reduce induced drag. The use of vertical ridges to encourage aftward flow does not significantly reduce outwardly directed spanwise flow and adds considerable drag.
U.S. Pat. No. 3,084,355 to Ciccotelli (1963) uses several narrow hydrofoils which rotate along a transverse axis and are mounted parallel to each other in a direction that is perpendicular to the direction of swimming. Although each hydrofoil has a substantially high aspect ratio, no system is used to adequately reduce induced drag.
U.S. Pat. No. 3,411,165 to Murdoch (1966) displays a fin which uses a narrow stiffening member that is located along each side of the blade, and a third stiffening member that is located along the central axis of the blade. Between the three members is a thin flexible web that is baggy so that when the blade is moved through the water, the web fills to form two belly shaped pockets along the length of the blade. These pockets increase in depth towards the trailing edge. Other ramifications include the use of a solitary pocket, as well as a plurality of such pockets.
A major problem with these designs is that the angle of attack is high and significant back pressure develops within each pocket. Although it is intended that the water is to be channeled towards the trailing edge, this does not efficiently occur. Because the water is striking the blade""s webbing at a substantially high angle of attack (close to 90 degrees), the water resists making a sharp change in direction and is not efficiently accelerated toward the trailing edge. Consequently, the relatively large volume of water attempting to enter the pocket soon backs up and spills around the side edges of the pocket like an overfilled cup. This outwardly directed spanwise flow strengthens induced drag type vortices which further drain water from the pocket. Only a small amount of water is discharged aftward and propulsion is poor. No method is utilized to significantly decrease lee surface flow separation and induced drag.
French patent 1,501,208 to Barnoin (1967) employs two side by side blades which are oriented within a horizontal plane and extend from the toe of the foot compartment. The two blades are separated by a space between them. A vertically oriented blade is mounted to the front portion of the foot compartment and is located within the space existing between the two blades. This vertical blade is relatively thin and extends above and below the plane of the horizontal blades as well as a significant distance in front of the toe.
This vertical blade does not significantly contribute toward propulsion. It also adds drag and blocks water from flowing between the horizontal blades. Its extension below both the blades and the foot compartment make the fin difficult to walk on across land or stand up while in the water.
The most significant problem with this design is that the structure of each horizontally aligned blade prevents it from significantly twisting about an axis that is substantially parallel with its length. No structure is offered to encourage such twisting to occur in an efficient manner. In addition, no mention is given to suggest a need for such twisting. As a result, the blades stall through the water during use.
Although each blade is made of flexible material, its structure creates stresses within the blades"" material which prevent the blades from achieving a substantially twisted shape along their lengths during kicking strokes. If any twisting forces are applied to the blades during use, significantly high levels of torsional stress forces occur in the form of tension and compression within the blades"" material. These stress forces occur diagonally across the entire length of each blade. As a result, a large volume of each blade""s material must succumb to these forces before any twisting can occur. A simple bending motion across each flexible blade places a much smaller volume of each blade""s material under the influence of tension and compression forces than that would occur during a twisting motion. Consequently, the exertion of water pressure causes the blade to bend backwards around a substantially transverse axis under the exertion of water pressure created during use before it can begin to attain a twisted shape around a substantially lengthwise axis.
Although Barnoin""s end view drawing shows that the blades taper in a sideways direction from the outer side edge toward the inner side edge, the blades remain highly resistant to twisting around a lengthwise axis. Barnoin does not state that the inner side edges of each blade should be more flexible than the outer side edge. However, even if it is assumed that the tapered inner side edge is more flexible, only a significantly small amount of flexing occurs because each blade tapers in a uniform manner from its outer side edge to its inner side edge. Such uniform tapering causes the resistive forces of tension and compression to be exerted over an increased volume of material within each blade. This is because the cross sectional thickness of the blade is significantly thick over most of its span. This substantially increases each blade""s resistance to bending around a lengthwise axis. Also, as each blade bends back under water pressure around a transverse axis, each blade becomes arched across its length. This makes each blade even more resistant to bending around a lengthwise axis.
These torsional stress forces existing within each blade that inhibit twisting occupy a significantly large portion of each blade""s material, and no adequate system or structure is used to control these stress forces in a manner that permits the blades to twist around a significantly lengthwise axis. In Barnoin""s design, these stress forces are strongest on an area of each blade that exists behind (toward the foot pocket) an imaginary line which originates substantially from the root portion of each blade""s inner side edge near the foot pocket and extends to a point on each blade""s outer side edge that is about half way between the root and the trailing edge. The imaginary line actually originates at a position along the inner side edge that is approximately one third of the way between the foot pocket and the trailing edge. This is because the tapered spanwise cross sectional shape of each blade transfers anti-bending stress forces from the thicker outer side edge to the thinner inner side edge, thereby artificially stiffening the inner side edge of each blade. This imaginary line then extends approximately to the mid-way portion of each blade""s outer side edge because the outer half of each blade is shown and described as tapering significantly along its length and becoming highly flexible about half way between the root and the trailing tip. Between this transversely directed imaginary line and the foot pocket, each blade is plagued with high levels of stress forces which prevent this area from twisting during kicks. This causes flow separation and stall conditions to occur along the low pressure surface of these blade portions.
The areas of each blade which are forward (toward the trailing edge and away form the foot pocket) of this imaginary line are much less effected by these stress forces. If each blade is made from a highly flexible material, then each blade bends around this transversely directed imaginary line. This causes the portions of each blade between this imaginary line and the trailing edge to deform to a reduced angle of attack by bending around a substantially transverse axis which is substantially parallel to the imaginary line. Because this axis is slightly swept back, the outer portions of each blade bend in a slightly anhedral manner. However, this anhedral angle is not sufficiently anhedral enough to create any significant reductions in lee surface flow separation, induced drag, or outward spanwise cross flow conditions. This is because the blades are bending around a highly transversely directed axis. In addition, when highly flexible materials are used in this design, the outer half of each blade collapses to a zero, or near zero angle of attack. This creates high levels of lost motion between strokes and does not permit significant levels of lift to be generated.
Another problem not anticipated by Barnoin is that if the two separate blades are permitted to deform in a slightly anhedral manner, a small amount of water can be deflected toward the space between the blades. This inwardly defected flow creates an equal and oppositely directed force against each blade which pushes outward on each blade in a spanwise direction. As a result, the portions of each blade existing between the imaginary line and the trailing edge spread apart a significantly large distance from each other and collapse to an excessively low angle of attack. Barnoin does not mention that he is aware of any such outward spanwise deformation of the blades and does not describe a method or structure that is capable of effectively controlling this undesirable occurrence.
As each blade pair spreads apart from each other on each of the users feet, the overall span of each swim fin increases substantially. This can cause the swim fin on one foot collide with the swim fin on the other foot as the swim fins pass each other during use in a scissor-like kicking stroke. In addition, much of the energy created by the kicking motion is wasted because it is used to spread the blades apart rather than propel the swimmer in a forward direction. Significantly high levels of lost motion also occur during the time that the blades are spreading apart at the beginning of each stroke, as well as when they are coming back together at the end of each stroke. This combines with the lost motion occurring as each blade bends backward around a transverse axis. The stress on each blade created by this spreading motion also causes each blade to collapse to an excessively low angle of attack that is incapable of producing significant levels of lift.
Because no structural solution to these problems are mentioned, the only way that this spreading motion can be controlled within the confines of Barnoin""s design is to make the blades out of a more rigid material. This only further increases each blade""s resistance to twisting or flexing around a lengthwise axis. Consequently, using a more rigid blade causes a larger portion of each blade""s surface area to suffer from stall conditions, induced drag vortex formation, and inadequate lift generation just as making the blades out of a more flexible material causes a larger portion of each blade to bend backward around a transverse axis to an excessively low angle of attack which is incapable of generating significant levels of lift. Either way, serious problems result which destroy performance.
If Barnoin""s design is made with sufficiently rigid enough blades to avoid excessive levels of lost motion and spanwise spreading, the spanwise tapering of the blades causes the anti-bending stress forces at the outer side edges of the blades to be transferred to the inner side edges of the blades. This stiffens the inner side edges of each blade and prevents them from deforming significantly under water pressure. As a result, a significant difference in rigidity does not exist between the outer side edges and inner side edges of the blades. This prevents the blades from bending around a significantly lengthwise axis.
If any flexing occurs during use on such rigid blades, it can occur only on an insignificantly small portion of each blade""s inner side edge. Because the cross sectional shape of this design transfers anti-bending stress forces from the outer side edge to the inner side edge of each blade, the majority of each blade""s spanwise alignment remains at excessively high angles of attack. This permits high levels of flow separation to occur as water spills around the outer side edges of each blade. This stalls the blades and produces high levels of drag from induced drag vortices and transitional flow. In addition, the transference of this stiffening effect to the inner side edge of each blade causes the inner side edge of each blade to also be at an excessively high angle of attack. This causes high levels of flow separation to occur at this location. As a result, significantly strong induced drag vortices form along the inner side edge and outer side edge of each blade""s lee surface. This creates high levels of drag and inadequate levels of lift.
German patent 259,353 to Braunkohlen (1987) suffers from many of the same problems and structural inadequacies as Barnoin""s fin discussed above. Braunkohlen uses a wedge like incision along the fin""s center axis which leads from the trailing edge of the fin to a small circular recess near the toe area of the foot pocket. This incision divides the blade region into left and right blade halves. Each blade half decreases in thickness from its outer side edge to its inner side edge (the incision side of each blade half) to make the blade continuously weaker toward the incision. The tapering reaches a uniform thickness along the incision side of the blade.
Gradation markings in the drawing show that each blade also decreases in thickness and strength from the base of the blade (near the foot pocket) towards its trailing edge which is extreme end of each blade located in front of the foot pocket. These gradation markings show that a significantly large portion of each blade""s trailing portion is as thin and structurally weak as the inner edge of each blade bordering the incision. This causes a significantly large portion of each blade""s surface area to be highly vulnerable to excessive deformation around a transversely aligned axis. This type bending creates an arched contour around this a transverse axis which significantly increases each blade""s resistance to twisting around a significantly lengthwise axis. No adequate structure is offered by Braunkohlen to compensate for this occurrence.
Because Braunkohlen""s blades are highly vulnerable to bending around a transverse axis, a substantially large portion of each blade""s surface area can bend to a zero or near zero angle of attack during use. At such low angles of attack, the blades are inefficient at generating significant levels of lift. High levels of lost motion occur as the blades xe2x80x9cflopxe2x80x9d loosely back and forth at the inversion point of each alternating stroke. As a result, much of the energy used to kick the blades through the water is used up deforming the blades to inefficient orientations rather than being converted into propulsion.
Because no adequate structure is shown to significantly reduce this problem, the only way to reduce lost motion is to make the blades out of a sufficiently rigid enough material to prevent excessive levels of bending around a transverse axis from occurring during strokes. By making the blades out of a stiffer material, high levels of stress forces are allowed to build up within each blade""s material. Because the blades taper in a uniform manner from outer side edge to inner side edge, these stress are transferred to the weaker portions of the blade bordering the incision. This significantly stiffens the inner side edge of each blade and prevents a significant portion of each blade near the incision from flexing when water pressure is applied during strokes. This prevents each blade from bending or twisting about an axis that is substantially parallel to the lengthwise alignment of each blade. This stiffening effect causes a significantly large portion of each blade""s outer side edges to remain at an excessively high angle of attack during use. This causes high levels of separation to occur as the water passes around each blade""s outer side edge. In addition, the transference of this stiffening effect to the inner edge of each blade bordering the incision causes the inner side edge of each blade to also be at an excessively high angle of attack. This causes high levels of flow separation to occur at this location. As a result, significantly strong induced drag vortices form along the inner side edge and outer side edge of each blade""s lee surface. This creates high levels of drag and inadequate levels of lift.
Also, Braunkohlen does not anticipate that any significant amount of deformation along the inner side edge of each blade half deflects water toward the incision and thus creates an outward spanwise force on each blade half. If the blades are flexible enough to permit significant deformation to occur near the incision, this outward force causes the blade halves to spread apart from each other during use. Braunkohlen does not mention a method for effectively countering this outward force and no adequate structural system is provided for controlling or reducing such spanwise spreading. As a result, this design is vulnerable to high levels of lost motion as the blade halves spread apart from each other at the beginning of each stroke and coming back together at the end of each stroke. Also, the energy expended in deforming the blades in a spanwise direction is wasted since it is not converted into propulsion.
Another problem with this design is that while the blades are spreading apart from each other, each blade buckles under stress and bends around a substantially transverse axis. This is largely because the trailing portions of each blade are much weaker and more flexible than the leading portions of each blade. This causes a significantly large portion of each blade to bend to an excessively low angle of attack which is inefficient at generating lift.
Because no structural features are used to efficiently overcome these problems and exert control over each blade""s shape, any attempt to merely change each blade""s flexibility cannot not significantly improve performance. While an increase in rigidity causes more of the fin""s surface area to remain at an excessively high angle of attack, an increase in flexibility only increases the tendency for each blade to bend backward around a transverse axis and spread apart from each other in a spanwise direction. In either situation, flow separation is high and lift is low.
The circular recess at the base of the incision is shown to be relatively small and only slightly larger than the narrow incision. Braunkohlen states that it""s purpose is to prevent the base of the incision from tearing during use. Also, the span of the circular recess is proportionally too small for it to have any other benefit to performance. The elevated section behind the recess is also used only to reinforce the base of the incision so that the fin is less likely to tear along the center axis.
French patent 1,501,208 to Barnoin (1967) also displays a differently configured alternate embodiment which uses four blades attached to one foot compartment. An end view drawing from the tips of the blades illustrates that the four blades are arranged in a cross sectional configuration that is substantially X-shaped. This orientation places the four blades within two diagonal planes which cross each other at the fin""s center axis. The blades are spaced apart from each other to form a gap at the middle of the X-configuration. The drawing reveals that each blade tapers in thickness towards this gap to form a sharp inner side edge and a thicker outer side edge.
The X-configuration of the blades is highly inefficient and causes excessive drag while kicking because the trailing blades on each stroke prevent the leading blades from efficiently generating lift. When the fin is kicked upward, the upper pair of blades are the leading blades and the lower pair of blades are the trailing blades. When the fin is kicked down, the opposite occurs. Although in both situations the leading blades are angled in anhedral manner to offer a reduced angle of attack, the trailing blades are always angled in a dihedral manner that prevents the leading blades from generating lift. Because the trailing blades are positioned at an extremely high angle of attack relative to the water curving around the outboard edges of the leading blades, the path of water traveling along the low pressure surfaces of the leading blades becomes blocked by the orientation of the trailing blades. This prevents the water curving around the lee surface of the leading blades from efficiently joining the water that is leaving the attacking side of the leading blades at the inner side edge of the leading blades. This prevents the formation of a significantly strong a low pressure zone along the lee pressure surface of the leading blades, and therefore prevents significant levels of lift from being generated.
The high angle of attack of the trailing blades also increases induced drag vortex formation around the outer side edges of the leading blades by creating a pocket on each side of the fin between the leading and trailing blades. The induced drag vortex becomes trapped, protected, and amplified within this pocket. The separation created by this vortex completely stalls each leading blade. This creates high levels of drag and destroys lift. In addition, the swirling eddy-like motion of this trapped induced drag vortex causes the water flowing along the lee surface of the attacking blades to flow backward from the inner side edge toward the outer side edge. This backward directed flow created by this eddy-like swirling motion is highly undesirable since it occurs in the opposite direction of what is needed to generate lift on the leading blades.
This undesirable eddy also reverses the direction of expected flow along the attacking surface of the trailing blades so that water along these surfaces flow from the outer side edge toward the inner side edge on each blade. This prevents lift from being generated by the trailing blades as well.
Other problems of this design occur as the flexible blades deform in an uneven manner during kicking strokes. When water pressure is exerted against the leading pair of blades, the flexibility of these blades enable them to bend backward around a transverse axis and press against the trailing blades. Because the trailing pair of blades are not exposed to the oncoming flow, they remain relatively straight while the leading blades push against them. As the inner side edges of the leading blades contact the inner side edges of the trailing blades, the path of water traveling along the low pressure surfaces of the leading blades becomes completely blocked so that it cannot merge with the water leaving the attacking side of the leading blades at the inner side edge of the leading blades. This prevents a low pressure zone from forming along the low pressure surface of the leading blades, and therefore prevents lift from being generated.
Although the leading pair of blades are anhedrally oriented in a manner that can encourage water to flow toward the void existing between the two leading blades, no method or structure is discussed for countering the spanwise directed outward forces exerted upon each blade by such inward flowing water. Because the blades are flexible and vulnerable to this outward force, they spread apart from each other in a transverse direction. This wastes energy, creates lost motion, and produces awkward blade orientations that inhibit performance.
In addition to offering poor levels of performance, this arrangement of four blades increases production costs through increased materials, parts, and steps of assembly. Also, both the added weight and bulk increase the cost of packaging, shipping, and storage. Such added weight and bulk inconveniences the user as well.
U.S. Pat. No. 3,934,290 to Le Vasseur (1976) uses a single fin which receives both feet of the user for use in dolphin style kicking strokes. Because no system is used to reduce outwardly directed spanwise flow along the attacking surface of the blade near the tips, this design is subject to high levels of induced drag.
Le Vasseur uses a series of vents which are aligned in a spanwise direction. The passage ways of these vents extend from above the toe of the foot pockets diagonally through the blade to a line near the trailing edge on the underside of the blade. This orientation only permits the vents to be used on the down stroke. These vents do not significantly reduce the creation of induced drag.
U.S. Pat. No. 4,007,506 to Rasmussen (1977) uses a series of rib-like stiffeners arranged in a lengthwise manner along the blade of a swim fin. The ribs are intended to cause the blade to deform around a transverse axis so that the trailing portions of the blade curl in the direction of the kicking stroke. The blade employs no method for adequately decreasing induced drag. The blade""s high angle of attack stalls the blade and prevents smooth flow from occurring along its low pressure surface.
The ribs are not intended to encourage the blade to twist or bend in a manner that decreases separation along the low pressure surface of the blade. Instead, the ribs prevent the blade from bending to a lower angle of attack. Rasmussen""s uses ribs in an attempt to increase the angle of attack existing at the outer portions of the blade.
U.S. Pat. No. 4,025,977 to Cronin (1977) shows a fin in which the blade is aligned with the swimmers lower leg. This design is highly inefficient on the upstroke. No system is used to reduce the presence of induced drag.
U.S. Pat. No. 4,521,220 to Schoofs (1985) uses a fin designed for use by breast stroke swimmers. It employs a horizontal blade with a transversely directed asymmetric hydrofoil shape. The design is stated to be stiff enough to hold its shape during swimming. This prevents the fin from being effective when used in a conventional up and down scissor-like kicking stroke. This is because the hydrofoil shape is perpendicular to the direction of such strokes. This causes the blade to stall. Even during breaststroke kicking styles, no system is employed to significantly reduce induced drag.
U.S. Pat. No. 4,541,810 to Wenzel (1985) employs a single fin designed to be used by both feet in a dolphin style kicking motion. The design uses a stiff, load bearing Y-shaped frame member, and a highly resilient webbing secured between the forks of the frame The web is intended to cup the flowing water by arching its surface as the forks flex inward in response to the water pressure placed on the web during strokes.
This method of creating a cup to channel water toward the center of the fin and out the trailing edge is highly inefficient since it quickly builds up excessive back pressure within the webbing""s pocket. This back pressure reverts flow back over the outboard side edges of the fin like an over filled cup. This increases the formation of induced drag vortices along the low pressure surface along these side edges. These vortices create drag, decrease lift and quickly drain the high pressure center occurring in the arched pocket. Because a significantly large portion of the water flowing along the attacking surface spills sideways around the outer side edges of the hydrofoil, forward propulsion is poor and drag is high.
Another problem is that as the webbing bows under water pressure, it forms a parabolic shape in which the outer side edges of the webbing experiences the least amount of curvature and the center regions of the webbing experience the greatest amount of curvature. This type of parabolic shape occurs whenever an evenly distributed load is applied to a material that is suspended across a surrounding frame. This parabolic shape cause the outer edges of the webbing near the frame member to remain at an excessively high angle of attack relative to the oncoming water. The high angles of attack exhibited by the leading and side edges of the blade also create separation and stall conditions along the low pressure surface of the blade which further reduce lift and increase drag.
Although some of Wensel""s embodiments show a deep V-shaped cut-out section along the trailing edge, no system is used to control the shape of these trailing portions as they deform. The cut-out along the trailing edge consists of two concavely curved outer portions existing near the tips, as well as two convexly curved inner portions which meet at the center of the webbing to form a small and narrow V-cut which ends in a sharp point. An imaginary straight line extending from a tangent of each concave outer portion to the sharp point of the V-cut at the center of the trailing edge, is the rearward limit (toward the trailing edge) of the spanwise tension forces which occur across the resilient webbing. The region of the webbing existing between this imaginary line and the forked frame are highly resistant to twisting around a lengthwise axis. This is because this region is plagued with anti-twisting stress forces of compression and expansion. On the other hand, the portions of the webbing which exist between this imaginary line and the trailing edge are structurally weaker than the rest of the webbing because this area is significantly less affected by the tension forces occurring across the resilient webbing which are created while bowing under water pressure. As a result, the convex portions of the trailing edge region tend to fold substantially along this imaginary line to a significantly lower angle of attack than the rest of the webbing during use. This creates an abrupt change in the webbing""s contour and causes significant drag and loss of lift. Wenzel uses no system to support this zone. Because his webbing is highly resilient and easily deformable, it is especially vulnerable to this problem. The use of a more rigid material for the webbing only further inhibits the webbing""s ability to bow under water pressure.
Another problem with his design is that the forked ends of the stiff load bearing frame member will not adequately flex inward enough to create significant results. If the forked portions of the frame member are made strong enough to substantially maintain its lengthwise alignment during strokes and not bend excessively backward around a transverse axis under the exertion of water pressure, it will not be flexible enough to permit significant flexing to occur in an inward spanwise direction. This is primarily because the spanwise tension across the webbing, which is responsible for causing the forked ends of the frame to flex inward, is significantly less than the force created by drag which pushes backward against the forks in a direction that is opposite to the direction in which the fin is kicked through the water. This problem is further increased because the forks have a spanwise hydrofoil shape that causes each fork act like a sideways I-beam which is significantly more resistant to horizontal flexing (spanwise flexing) than to vertical flexing (backward bending around a transverse axis). If the forks are flexible enough to bend sufficiently inward to form a pocket in the webbing, they will not be rigid enough to avoid excessive backward bending (opposite to the fin""s direction of stroke) around a transverse axis to an excessively low angle of attack during use.
The structure of the forks also prevents them from experiencing significant levels of twisting during use. When twisting forces are applied to the forks, high levels of torsional stress forces build up within the fork""s material. In order for twisting to occur, the material must succumb to these stress forces and undergo significantly large amounts of expansion and compression across a majority of its length and volume. Since a significantly large portion of the fork""s material is forced to experience relatively high levels of compression and expansion, resistance to such twisting is significantly high. In comparison, a simple bending motion around a transverse axis permits significantly reduced levels of compression and expansion to occur over a significantly smaller portion of the fork""s material. As a result, solid objects many times less resistance to bending along the length than to twisting about their length. Because of this, the forks will not adequately twist during use in an amount sufficient to significantly reduce stall conditions and flow separation along the edges of the hydrofoil. This causes the hydrofoil shaped forks to remain at an excessively high angle of attack during use, thus creating further drag and loss of lift.
If the forks are made from a sufficiently resilient material to permit a significant amount of twisting to occur, it will bend backward and collapse around a transverse axis because the comparative resistance to such deformation is many times lower than that created during a twisting motion. In addition, the forces which attempt to twist the forks along their length (created from tension across the webbing), are significantly weaker than the forces created by drag on the hydrofoil which attempt to bend the forks backward in the opposite direction of the blade""s motion through the water.
If the forks are rigid enough to withstand the force of drag on the fin without excessive deformation, than they are not flexible enough to twist significantly along their length. Because of this, the spanwise hydrofoil shape of each fork remains at a high angle of attack during use. This creates high levels of flow separation along the lee surface of the fork during use. This increases induced drag vortex formation, stall conditions, and transitional flow. Because the leading edge portions of the fork also remain at an excessively high angle of attack, the leading edge of the hydrofoil stalls as well. As a result, drag is high and lift is poor.
U.S. Pat. No. 4,738,645 to Garofalo (1988) employs a single blade which deforms under water pressure to form a concave channel for directing water toward the trailing edge. The blade uses two narrow and lengthwise directed strips of flexible membrane located near the stiffening rails on each side edge of the blade. Between the two narrow strips of flexible membrane is a stiff and centrally located blade portion which is attached to the inner side edges of the two membrane strips. When the fin is kicked, water pressure pushes against the stiff central blade portion which applies tension to the flexible strips. As this occurs, a loose fold within each flexible strip elongates, thereby enabling the central blade portion to drop so that fin forms a scoop like channel.
Although this shape is intended to reduce flow around the sides of the blade and increase aftward flow, it does not do so efficiently and suffers from high levels of drag. Because the blade""s central portion is at a significantly high angle of attack, the water""s inertia resists a quick change in flow direction as it strikes the blade""s central portion. This creates a significant amount of back pressure within the channel. Because this design lacks a method for reducing such back pressure, the water backs up within the channel and spills sideways around the side edges of the blade like an overflowing cup. As this happens, the flow separates from the blade""s low pressure surface. This increases induced drag and destroys lift. The vertical ridges along the side edges of the blade do not efficiently reduce this problem and only add extra drag of their own.
Another problem is that the portion of the blade that lies between the side rails and the flexible strip is relatively wide and has significant torsional stress forces within it which prevent it from twisting significantly along its length during strokes. As a result, this portion always remains at a high angle of attack which increases the strength of induced drag vortices. Both the central and side portions of the blade remain at a high angle of attack which stalls the fin. This depletes lift and further increases drag.
U.S. Pat. No. 4,781,637 to Caires (1988) shows a single fin designed to be used by both feet in a dolphin style kicking motion. It uses a transversely aligned hydrofoil that extends from both sides of a centrally located foot pocket. The hydrofoil is made of a flexible material which has a stiffening rod located within it that runs parallel with the hydrofoil""s leading edge. The flexible material is loosely disposed around the stiffening rod to permit rotation. A plate-like member is located within the central portion of the hydrofoil to prevent the blade from rotating around the stiffening rod at this location.
Although the tips are intended to twist about the rod to a reduced angle of attack while the center region remains at a high angle of attack, the centrally located plate-like member introduces stress forces within the hydrofoil""s flexible material that strongly oppose such twisting. When water pressure applies a twisting force against the hydrofoil, torsional stresses of compression and tension build up within the flexible material in directions that are diagonal to the axis of rotation. While compression forces exist along one diagonal direction, tension forces exist along another direction that is substantially perpendicular to the direction of compression. This creates a complex network of stress forces within the flexible material between the plate-like member and the outer tips of the stiffening rod. Resistance to twisting is high because these forces are exerted across significant distances, and therefore large volumes of the flexible material must experience significant amounts of expansion and compression before twisting can occur. Because no adequate method is used to reduce these stress forces within the blade""s material, the blade demonstrates high levels of resistance to any twisting forces created by water pressure.
This is a major problem since the twisting force created by water pressure during strokes is significantly small. If the hydrofoil cannot twist quickly and substantially under conditions of significantly light pressure, the blade remains at an excessively high angle of attack which causes flow separation to occur along the lee surface thereby stalling the hydrofoil. When the flow quickly separates from the low pressure surface in this manner, the twisting force created by the water pressure drops off dramatically. Because the resistance to twisting is at a high, and the twisting force provided by water pressure is significantly low, the blade remains at a high angle of attack. This destroys lift and creates high levels of drag. Caires does not mention that he recognizes these problems created by torsional stress forces and offers no solution for controlling them.
Another problem with this design is that a much of the hydrofoil""s flexible material is poorly supported by the stiffening system. This makes the foil vulnerable to bending forces which can adversely deform the foil""s shape during use. The areas that are most vulnerable to such bending forces are located aft (towards the trailing edge) of an imaginary line which extends from each outboard tip of the stiffening rod, to the trailing portion of the centrally located stiffening plate. The areas between this imaginary line and the trailing edge bend abruptly to a reduced angle of attack. This bending occurs along an axis that is substantially parallel to this imaginary line.
This abrupt change in contour creates an undesirable cross sectional hydrofoil shape that causes the low pressure surface to become concavely curved, and causes the attacking surface to become convexly curved. According to Bernoulli""s principle, this shape reduces lift because it decreases the distance that the water must travel along the low pressure surface, while it simultaneously increases the distance that the water must travel along the high pressure surface (attacking surface). This reduces the overall difference in pressure existing between the low pressure surface and the attacking surface. In addition, the concavely curved low pressure surface formed during strokes also encourages the flow to separate from this surface. This further decreases lift and increases drag. While the trailing portions of the foil bend in this manner during use, the leading portions of the foil existing between the imaginary line and the leading edge remain at a high angle of attack because of the anti-twisting stress forces which exist in this region. This is highly inefficient because it stalls the leading portion of the blade.
Because of the structural inadequacies of this design, any attempts to merely change the resiliency of the blade can not significantly improve performance. If highly flexible materials are used to make the hydrofoil blade, the portions of the blade existing aft of the imaginary line collapse completely to a zero, or near zero angle of attack. This dramatically reduces leverage on the hydrofoil, and therefore reduces the twisting force created by the water pressure. Thus, even with highly flexible materials, the entire leading edge remains in a stall position during strokes. This destroys lift and creates drag.
Although the use of stiffer materials can reduce the abruptness and degree of this bending tendency, it also causes a larger portion of the blade to remain at an excessively high angle of attack. This is because less flexible materials permit the stiffening effect of the anti-twisting stress forces (present in the leading portions of the foil) to extend farther out towards the trailing edge. A major dilemma thus results: if the flexible material within the hydrofoil is resilient enough to twist under extremely light pressure its trailing portions collapse to an excessively low angle of attack during use; however, if the flexible material is sturdy enough to prevent the inadequately supported trailing portions from bending excessively, the material is no longer resilient enough to twist sufficiently under significantly light pressure. As a result, this design is highly inefficient.
Another problem displayed by the drawings is that the stiffening system within the leading edge of the hydrofoil does not extend far enough toward the outer tips of the hydrofoil. This permits the highly resilient material at the tips to flex in an uncontrolled and undesirable manner when the fin is kicked through the water. Significantly large areas of improperly supported resilient material are able to bend to an orientation that produces significant turbulence and drag. This is especially a problem at the outer side edges because the outboard flow conditions produced by induced drag vortices force the unsupported tips to bend dihedrally, along a chordwise axis. This encourages outwardly directed flow and therefore increases the strength of induced drag vortices. No method is employed to adequately reduce the formation of induced drag vortices.
The same problem is seen in the design which places the blades in a slightly swept back configuration. Lack of adequate support along the outer edges of the tips, permit the flexible material, which extends aft of the ends of the stiffening rod, to bend along a transverse axis. At the same time, dihedral bending occurs at the outboard ends of the flexible material because the span of the stiffening rod is significantly smaller than the span of the hydrofoil.
In the swept back version of his design, the blade-halves are not swept back enough to encourage a significant inward directed flow from occurring along the attacking surface of each blade-half. Although the extreme outer edges of the blade are significantly swept, these highly swept portions of the blade are not properly supported and therefore encourage outward spanwise directed flow to occur along the attacking surface near the tips of each blade-half.
Another problem with this design is that the significantly high aspect ratios that Caires uses causes the spanwise dimensions to be significantly wide. This greatly reduces the ability of the swimmer to use this design in confined areas such as narrow passageways, arches, ravines, caves, kelp forests, and ship wrecks. Such wide spanwise dimensions also prevent this design from being used on separate fins for each foot for use in a scissor-like kicking stroke since the fin on one foot can collide with the fin on the other foot during use.
An alternate embodiment shows a cross sectional view of a hydrofoil having a chordwise linkage member suspended within a hollow hydrofoil made from a resilient plastic skin. The leading portion of this member is pivotally linked to a transverse stiffening member located within the leading edge of the hydrofoil. The trailing portion of the linkage member extends rearward and attaches to the inside of the trailing edge of the hollow hydrofoil. The only connection between the linkage member and the hollow skin is at the trailing edge. All other portions of the skin are free from the linkage member.
The sole purpose of this linkage member is to create a variance in skin tension between the upper and lower surfaces of the hollow hydrofoil so that an asymmetrical shape is created during use. The chordwise linkage members are not used, or intended to be used in a manner that can relieve or control anti-twisting stress forces that are created within the blade""s material during use. This prevents the hydrofoil from achieving a smooth and efficient contour when twisting forces are applied to the blade.
Because of the structure of this design, the loosely disposed skin tends to buckle and wrinkle when anti-twisting stress forces of compression and tension build up within it during use. Because these stress forces are created diagonally across the span of the skin, diagonally directed wrinkles form across the upper and lower slices of the hydrofoil. These wrinkles can be observed forming when one end of a hollow object such as a water bottle (semi-filled with either water or air) is twisted while the opposite end is held stationary. Because the skin on the upper and lower surfaces is loosely disposed above and below each linkage member within the hydrofoil, this buckling tendency cannot be controlled by the linkage members. The greater the degree of spanwise twisting, the greater the degree of buckling and wrinkling within the skin. The resulting wrinkles create turbulence and separation. This destroys lift and creates high levels of drag. Also, because two separate skins are used (upper surface and lower surface) twice as much resistance to twisting (from tension forces) results than if only a single membrane is used.
U.S. Pat. No. 4,857,024 to Evans (1989) shows a fin which has a flexible blade with a V-shaped cut along the trailing edge. The blade does not form an anhedrally oriented channel along the attacking surface of the blade during strokes. The V-shaped cut along the trailing edge only extends a relatively small distance in from the trailing tips and does not cover a significant length of the blade. Because of this, the V-shaped cut is not in a position for significantly preventing excessive back pressure within the fluid existing along the center regions of the blade.
The blade is thickest and most rigid along its center axis. The blade decreases in thickness on either side of this center axis toward its side edges for increased flexibility near these edges. The center axis of the blade lies in the same horizontal plane as the foot pocket, while the portions on either side of the center axis angle upward toward the outer side edges. These angled portions form a convex up V-shaped valley. When this upper surface is kicked forward the outer portions start out in an anhedral orientation relative to the direction of movement. However, as soon as water pressure is applied against these upwardly angled outer portions, these portions flex back into alignment with the horizontal plane of the center axis, and then continue to flex beyond this point to assume a dihedral orientation during this upwardly directed kicking stroke. At this point, the stiffer central portion of the blade arches back around a transverse axis to an excessively reduced angle of attack where the blade then slashes back at the end of the stroke in a snapping motion to propel the swimmer forward.
This snapping motion acts more like a paddle than a wing, rather than creating lift like a wing, this design snaps backward at such a high angle of attack that no smooth flow can occur along the lee surface of the blade. Consequently, this snapping motion attempts to push the swimmer forward by applying the stored energy within the backward bent blade against the drag that the blade creates within the water. This design creates significantly high levels of drag during use and causes significant levels of ankle fatigue. Also, the excessive backward deformation of the blade creates significant levels of lost motion during strokes.
On the opposite stroke where the lower surface of blade is the attacking surface, the angled outer ends are oriented at a dihedral angle relative to the direction of travel. The water pressure created during this stroke only increases this dihedral angle. This orientation directs water away from the center of the blade and toward the outer side edges. This increases induced drag and decreases lift. No system is used to create smooth flow conditions along the low pressure surface of the blade.
This design is especially difficult to use while swimming along the surface. Since the swimmer is usually face down in the water, the anhedrally oriented upper surface is also face down in the water. Because no system is used to reduce back pressure along the attacking surface of the blade, the anhedral blade acts like a parachute when re-entering the water. This brings the fin to an immediate stop as the blade strikes the surface. This transfers significant levels of strain to the user""s ankles and lower legs. The energy initially built up on the doffs stroke is wasted and new energy must be applied in order to regain movement.
U.S. Pat. No. 4,934,971 to Picken (1990) shows a fin which uses a blade that pivots around a transverse axis in order to achieve a decreased angle of attack on each stroke. Because the distance between the pivoting axis and the trailing edge is significantly large, the trailing edge sweeps up and down over a considerable distance between strokes until it switches over to its new position. During this movement, lost motion occurs since little of the swimmer""s kicking motion is permitted to assist with propulsion. The greater the reduction in the angle of attack occurring on each stroke, the greater this problem becomes. If the blade is allowed to pivot to a low enough angle of attack to prevent the blade from stalling, high levels of last motion render the blade highly inefficient.
Picken uses an elliptical shaped blade design in an effort to decrease induced drag. Because of its low aspect ratio and the significantly high angles of attack used during strokes, this design does not effectively reduce induced drag. In addition, no adequate method is offered for effectively discouraging outward flow along the side edges of the blade.
U.S. Pat. No. 4,940,437 to Piatt (1990) uses a swim fin blade that has a stiffening rod within the blade which runs along its center axis. This stiffening rod is not used in a manner that effectively reduces induced drag. No twisting motion is encouraged within the blade along a lengthwise axis.
Many of the same problems that exist with prior swim fin designs also exist in prior flexible propulsion blade designs that oscillate back and forth to generate propulsion. All such designs lack an efficient method for reducing induced drag and stall conditions. Designs that are intended to flex do not include an effective method for controlling or reducing undesirable stress forces within the blade that cause the blade to deform in an undesirable manner.
U.S. Pat. No. 144,538 to Harsen (1873) uses a series of pendulous arms driven by a rotating worm shaft to produce a wriggling or worm-like action. The system is dependent on a rotating worm shaft to provide shape. No system is used to reduce induced drag vortex formation along the submerged bottom edge of the blade system.
A book reference found in the United States Patent and Trademark Office in class 115/subclass 28 labeled xe2x80x9c3302 of 1880xe2x80x9d shows a horizontally aligned reciprocating propulsion blade. The planar blade has a narrow void existing along the center axis of the blade which divides the blade into two side-by-side blade halves. This void originates at the trailing edge of the blade and ends near the base of the blade. No system is used to encourage the blades to twist along a substantially lengthwise axis, and no system is used to encourage water to flow away from the outer side edges of each blade half. The blades only flex backward around a transverse axis in response to water pressure. Consequently, the blade stalls through the water and produces high levels of drag and poor propulsion.
Spanish patent 17,033 to Gibert (1890) shows a vertically aligned flexible oscillating propeller blade that has a triangular shaped void along its center axis that divides the blade into two blade-halves. The void is widest at the trailing edge and converges to a point at the base of the blade. No system is used to encourage the blade to twist or bend around a lengthwise axis. The blade-halves stall through the water and produce high levels of drag and poor levels of lift.
U.S. Pat. No. 787,291 to Michiels shows a vertically aligned oscillating propulsion system which has two blades with a space existing between them. Both blades lie within the same vertical plane. No system is used to permit the blades to twist along a lengthwise (chordwise) axis, and no system is used to reduce stalling or induced drag.
U.S. Pat. No. 871,059 to Douse (1907) shows a vertically aligned oscillating propeller which has a caudal shaped frame with a flexible membrane stretched between it. No adequate system is offered for reducing back pressure within the flexible membrane. As a result, outward spanwise cross flow conditions are created which decrease propulsion and increase induced drag. No system is used to reduce the membrane""s tendency to form a parabolic pocket when water pressure is applied. This parabolic shape causes the leading and side edges of the membrane to remain at a high angle of attack while the center region of the pocket becomes bowed. Consequently, the blade stalls and produces high levels of induced drag. In addition, the wide structure of the rigid frame member causes additional flow separation and drag.
U.S. Pat. No. 1,324,722 to Bergin shows a flexible oscillating propeller that has a narrow void along its center axis that divides the blade into two blade-halves. The void originates at the trailing edge and ends at a point near the base of the blade. The blade is made of a resilient material and is reinforced with a series of chordwise stiffening members which are joined to a transversely aligned stiffener a significant distance from the base of the blade. Because a significantly large portion of flexible blade material is unsupported along the outer side edges of the blade, these side portions deform in a dihedral manner under the exertion of water pressure. This increases outward spanwise flow conditions along the attacking surface of the blade. The stiffening members are not arranged in a manner that encourage the blade to deform in a manner that reduces such stall conditions and induced drag.
British patent 234,305 to Bovey (1924) uses propeller blades that have a fixed leading portion and a hinged trailing portion that swings freely along a substantially transverse axis. Because the trailing portion swings freely its inclination is uncontrollable. This allows this portion of the blade to bend backward under water pressure to an excessively low angle of attack. Consequently, sharp changes in contour can destroy efficiency and create drag. No system is used to effectively reduce induced drag.
U.S. Pat. No. 2,241,305 to Hill (1941) shows a vertically aligned propelling blade that uses a rigid frame which is shaped like the lower half of a caudal fin. A resilient membrane is stretched between the frame members. No system is used to reduce the membrane""s tendency to bow in a parabolic manner. Consequently, the edges of the membrane bordering the frame members remain at an excessively high angle of attack during use. This causes the blade to stall and produce high levels of induced drag.
U.S. Pat. No. 3,086,492 to Holley shows a vertically aligned oscillating propulsion blade that is made of a flexible material. The blade""s center axis has a V-shaped recess which divides the trailing portion of the blade into upper and lower halves. Paired stiffening ribs extend from both sides of the vertical blade in three locations. These blade pairs do not extend fully from the trailing edge to the base of the blade. Instead, a significantly large area of the blade""s flexible material exists between the leading ends of the ribs and the base of the blade. This lack of support renders the blade vulnerable to collapse around a spanwise axis.
The positioning of the rib pairs are also poorly organized. Although two of the rib pairs run parallel to the outer side edges of the blade, a significant distance exists between these rib pairs and the outer side edges of the blade. Consequently, a substantially large portion of the blade""s side edges are unsupported. This causes these edges to deform in a dihedral manner during use. This increases stall conditions as well as induced drag. The rib pair existing along the blade""s center axis only adds extra leverage to the bending forces which allow the blade to bend around a spanwise axis. This spanwise axis exists substantially along an imaginary line connecting the leading ends of each rib pair. The ribs are not arranged in a manner that encourages the blade to bend or twist around a substantially lengthwise axis. As a result, the blade stalls through the water and delivers poor performance.
U.S. Pat. No. 3,453,981 to Gause (1969) uses a series of horizontally aligned propulsion blades that are intended to convert wave energy into forward motion on a boat. Each blade has a space along its center axis that divides it into a left and right blade half. The most significant problem with this blade design is that it has no system for controlling the undesirable stress forces created within the blade""s flexible material during use. As a result, these stress forces prevent the blade from deforming in a desirable manner, and performance is poor.
Each blade has a rigid leading edge portion that is rounded and tapers gradually to a relatively resilient trailing portion. Although a dotted line in the diagram at first appears to represent a junction between these two areas of the blade, the description states that these two portions xe2x80x9cmerge smoothly into one another without any abrupt change in characteristic.xe2x80x9d Such a smooth transition and gradual tapering transfers anti-flexing stress forces aft on the blade (toward the trailing edge). Thus, the rigidity of the leading edge portion is extended a significant distance toward the more resilient portions of the blade. This prevents the more resilient blade portions from flexing significantly near the leading and side edges of the blade. Consequently, these leading and side edges remain at an excessively high angle of attack during use which causes the blade to stall. Strong induced drag vortices are permitted to form along the outer side edges and performance is poor.
Another problem with the structure of this design is that stress forces of compression and tension are permitted to build-up within the blade""s material during use. This prevents each blade half from adequately twisting along its length. These stress forces are strongest forward (toward the leading edge) of an imaginary line on each blade half that extends from the outer side edge of the extreme tip of the blade half to the most forward point of the trailing edge existing at the blade""s center axis. The strength of the anti-twisting stress forces prevent this portion of the blade from twisting along its length. This is because these stress forces are significantly strong in comparison to the water pressure applied during use. As a result, the leading portions of the blade to remain at an excessively high angle of attack which stalls the blade and increases induced drag.
The portion of each blade half that exists between this imaginary line and the trailing edge are less affected by these stress forces. Consequently, this portion of each blade half bends around an axis that is substantially parallel to this imaginary line. However, because the blade tapers gradually from the rigid leading portion to the more flexible trailing portion, the stress forces existing forward of this imaginary line are extended aft of the imaginary line. As a result, the blade deforms around an axis that is significantly aft (toward the trailing edge) of this imaginary line. Thus, only a small portion of the blade bends under water pressure. If the blade""s trailing portions are made from a significantly flexible material, the portions aft of the imaginary line collapse sharply under water pressure. In any case, the areas forward of this line remain in a stall condition which severely reduces lift.
Another problem occurs when the portions aft of the imaginary line bend back-ward from water pressure during use. As this happens, the swept alignment of each blade half causes some of the water traveling aft of this imaginary line along the attacking surface to be deflected toward the blade""s center axis. This inward deflection of water creates an outward spanwise force against each blade half. This causes the blade halves to spread apart from one another in a spanwise direction during each stroke. This destroys efficiency by creating high levels of lost motion and lost energy.
Gause does not anticipate this problem of spanwise spreading and offers no solution for avoiding it. Although he states that the leading portions of the foil are to be significantly rigid, he does not mention that it should be rigid enough to prevent this problem. If his design is made rigid enough to avoid this problem, the gradual tapering in the blade""s cross section extends this rigidity significantly toward the blade""s trailing portions. This causes the entire blade to be much too rigid to flex in a significant manner. Because no method is employed to control these problems, this design is highly inefficient.
U.S. Pat. No. 3,773,011 to Gronier (1973) shows a horizontally aligned propulsion blade having a forked frame and a flexible membrane stretched between the forks. The most significant problem with this design is that no system is used to reduce the occurrence of back pressure within the membrane""s attacking surface. As a result, back pressure causes the water along the attacking surface to spill in an outward spanwise direction around the side edges of the hydrofoil. This increases induced drag and severely inhibits propulsion.
Also, no method is used to control the membrane""s natural tendency to attain a parabolic shape as it bows out under water pressure. As a result, the greatest degree of bowing occurs near the center of the membrane near the trailing edge, while the leading and side portions of the membrane located near the forks experience only a minimal defection from the horizontal plane. This causes the water flowing around the leading and side edges of the hydrofoil to separate from the low pressure surface of the membrane. This stalls the blade, creates drag, and destroys lift.
Although Gronier shows a spanwise cross sectional drawing that depicts his membrane as being capable bowing in a substantially elliptical manner, this is not what actually occurs. It is well known that when an evenly distributed load is placed on a flexible material that is suspended across a frame, a parabolic shape results across the material. Even if the membrane is able to bow out a significantly large degree during use, the parabolic shape still causes the greatest amount of bulging to occur along the membrane""s center axis. This takes curvature away from the leading and side portions of the membrane and places them in a stall condition. Increased bowing also creates increased lost motion since a greater portion of each stroke is use to merely deform the membrane.
U.S. Pat. No. 4,193,371 to Baulard-Caugan (1980) shows a swimming apparatus that uses a vertically aligned caudal-shaped propulsion blade together with a caudal-shaped hydrofoil for reducing drift during use. Both the Propulsion blade and the xe2x80x9canti-drift memberxe2x80x9d are rigid and lack a system for reducing stall conditions and induced drag.
Japanese patents 61-6097 (A) to Fujita (1986) and 62-134395 (A), also to Fujita (1987) show a caudal-shaped propulsion blade which has a thin flexible membrane stretched across a forked frame. No system is used to relieve back pressure within the attacking side of the membrane and no system is used to reduce the membrane""s tendency to form a parabolic shape as it bows out during use. As a result, this design produces high levels of drag and low levels of lift.
My own U.S. patent application Ser. No. 08276407 to McCarthy filed Jul. 18, 1994 describes several methods for reducing induced drag on foil type devices. However, the designs shown which are capable of being used in reciprocating motion situations (where the angle of attack reverses itself) require the use of complex control devices to invert the foil""s shape. No system is shown that permits this inversion process to occur automatically and repeatedly in resilient swim fin applications and resilient propulsion blade applications.
Accordingly, several objects and advantages of the present invention are:
(a) to provide hydrofoil designs that significantly reduce the occurrence of flow separation their low pressure surfaces (or lee surfaces) during use;
(b) to provide swim fin designs which significantly reduce the occurrence of ankle and leg fatigue;
(c) to provide swim fin designs which offer increased safety and enjoyment by significantly reducing a swimmer""s chances of becoming inconvenienced or temporarily immobilized by leg, ankle, or foot cramps during use;
(d) to provide swim fin designs which are as easy to use for beginners as they are for advanced swimmers;
(e) to provide swim fin designs which do not require significant strength or athletic ability to use;
(f) to provide swim fin designs which can be kicked across the water""s surface without catching or stopping abruptly on the water""s surface as they re-enter the water after having been raised above the surface;
(g) to provide swim fin designs which provide high levels of propulsion and low levels of drag when used at the surface as well as below the surface;
(h) to provide swim fin designs which provide high levels of propulsion and low levels of drag even when significantly short and gentle kicking strokes are used;
(i) to provide resilient hydrofoil designs which offer significantly less resistance to twisting about their length than to bending across their length;
(j) to provide methods for substantially reducing the formation of induced drag type vortices along the side edges of hydrofoils;
(k) to provide hydrofoil designs which significantly reduce outward directed spanwise flow conditions along their attacking surface;
(l) to provide hydrofoil designs which efficiently encourage the fluid medium along their attacking surface to flow away from their outer side edges and toward their center axis so that fluid pressure is increased along their attacking surface;
(m) to provide methods for significantly reducing back pressure along the attacking surface a hydrofoil in a manner that significantly reduces the occurrence of outward directed spanwise cross flow conditions near the outer side edge portions of the hydrofoil;
(n) to provide methods for significantly reducing separation along the lee surface of reciprocating motion foils which are used at significantly high angles of attack, and
(o) to provide methods for controlling the torsional stress forces of tension and compression within the material of a flexible hydrofoil so that the material exhibits significantly reduced levels of resistance to twisting along its length.
Still further objects and objectives will become apparent from a consideration of the ensuing description and drawings.